About a decade ago, conditional replenishment encoding of video signals was developed by F. W. Mounts and others to take advantage of the fact that a significant portion of a picture represented by a video signal does not change in the time interval between successive frames. In U.S. Pat. No. 3,571,505 issued Mar. 16, 1971, F. W. Mounts describes a technique wherein intensity differences and address information is transmitted for only those elements of the picture which have changed by more than a predetermined threshold. Since that initial work, numerous improvements have been made in the technique used to detect changes, in the schemes used to transmit the addresses of the changed pels and, indeed, the use of similar or related techiques has dramatically expanded to other areas of picture processing such as color video systems.
Another technique, also designed to improve the efficiency with which pictorial information is encoded, involves a linear transformation of the data at the transmitter, and a corresponding inverse transformation at the receiver. Such transformations, usually accomplished to adjust the characteristics of the signal to be transmitted to the transmission medium or to take advantage of statistical tendencies exhibited by the data, include Hadamard, Haar, cosine and slant transforms, and several others. U.S. Pat. No. 3,984,626 issued to F. W. Mounts, A. N. Netravali and B. Prasada on Oct. 5, 1976, is exemplary of transform coders, and describes a technique in which one of the transform coefficients can be efficiently coded, usually as a function of the values of other coefficients.
The two techniques just mentioned, namely, conditional replenishment and transform coding, have been combined to yield a hybrid type of coding in an attempt to obtain the advantages of both. In hybrid coding, small blocks of picture elements are applied to a transformation circuit and the coefficients thus obtained are compared with stored coefficients for previously processed pel blocks. The previous coefficients serve as predictors for the coefficients of the present block, and only the prediction errors need be encoded for transmission.
In one specific hybrid scheme that has been studied, a one-dimensional block of picture elements (lying along a single scan line) is transformed and the coefficients DPCM encoded with respect to coefficients from a vertically displaced block such as that from the previous scan line. In other schemes, which employ intraframe and interframe coding, respectively, the coefficients for a two-dimensional block are compared with a horizontally displaced block in the same frame or a spatially corresponding block in a previous frame. Also illustrative of hybrid encoding systems is the work of J. A. Roese described in a paper entitled "Interframe Coding of Digital Images Using Transform and Hybrid Transform/Predictive Techniques", which appeared in the University of Southern California Image Processing Institute Report No. 700, dated June, 1976, and a paper by Noble, Knauer and Giem entitled "A Real Time Hadamard Transform System for Spatial and Temporal Redundancy Reduction in Television" which was published in the Proceedings of the International Telemetering Conference in October, 1973. Yet another transform encoding work, by H. W. Jones, Jr., is described in Vols. 87 and 119 of the Proceedings of the Society of Photo-optical Instrumentation Engineers at pages 2 and 91, respectively.
While the combination of the conditional replenishment and the transform coding techniques produces improvements in picture quality at a given transmission rate, or enables transmission of good quality pictures over a channel with reduced bandwidth, not all of the potential of hybrid coding has been achieved, since the two techniques are incompatible, at least in one sense. The transform technique is intended to represent the changes in the intensity values of all of the pels in each block in a different form, namely, as a series of coefficient values. If not all of the intensity values change, the entire series of coefficients still must be computed, since even one intensity change can effect all coefficients. This situation results in processing and transmission of extra information which, in turn, reduces encoder efficiency.
In view of the foregoing, it is the broad object of the present invention to improve encoding of pictorial information using linear transformations, whereby the amount of information needed to represent a picture of desirable quality is reduced. A specific object is to allow efficient coding of the transform coefficients which represent the intensity values of a block of picture elements when not all of the intensity values have changed from the previous picture frame.